Transposable elements - selection (more later). Several...

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Transposable elements are mobile genetic elements that can move from one part of the genome to another. Generally they have repeated sequences at their ends and code for protein(s) in the middle. In moving from one location to another they can cause mutations. If the element landed in the middle of the coding sequence of a gene, it most likely would lead to a frameshift mutation or introduce a stop codon , and knock out the function of that gene. Gene duplications can occur by unequal crossing over where gene families exist on the chromosome, homologous chromosomes may misalign and cross over (recombine). The daughter chromosomes include one with an extra copy and one with one fewer copies. Chromosome rearrangements can also be viewed as mutations. Classic cases: inversions were a section of the chromosome is inverted with respect to the "normal" chromosome. Drosophila polytene chromosomes show characteristic banding patterns and allow for easy recognition of inversions. A paradigm of natural
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Unformatted text preview: selection (more later). Several important consequences: Inversions can act as suppressors of crossing over in the heterokaryotype (= heterozygote for two different chromosomal types). An inversion does not prevent crossing over per se but the recombination products that result from a crossover within the inversion either have two centromeres and are pulled apart in division, or lack a centromere and are not transmitted. Only the unrecombined parental chromosomes are transmitted. How will the frequency of an inverted chromosome in a population affect it role as a suppressor of recombination? The more frequent the inverted type gets, it will be present in a "homokaryotypic" state and recombination will not be suppressed. If the "inverted" chromosome were fixed in the population (=100%) then we would no longer consider it "inverted"....
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